1. Field of the Invention
The present invention relates to a method of manufacturing a circuit device in which a circuit element is resin-sealed.
2. Description of the Related Art
As methods of resin-sealing a circuit element such as a semiconductor element, there are a method in which the circuit element is housed inside a casing member and a method in which the circuit element is resin-sealed with a sealing resin such as an epoxy resin. In recent years, the sealing method employing a resin sealing has been frequently used in view of productivity and the like.
In a process of resin-sealing a circuit element, the circuit element and the like are housed within a cavity of a mold, and a sealing resin in liquid form is then injected into the cavity to thereby resin-seal the circuit element. This technology is described for instance in Japanese Patent Application Publication No. Hei 11-340257.
This resin-sealing process is described with reference to FIGS. 11A and 11B. FIG. 11A is a cross-sectional view showing the resin-sealing process, and FIG. 11B is a cross-sectional view showing a configuration of a circuit device 200 as manufactured.
Referring to FIG. 11A, an island 202 having a semiconductor element 204 fixed on the top surface thereof is housed within a cavity 214 formed by bringing an upper mold half 224 and a lower mold half 226 into contact with each other. Also, a pod 220 that communicates with the cavity 214 via a runner 218 is formed in the lower mold half 226, and a tablet 228 is housed in the pod 220. The tablet 228 is formed by pressure-molding a granular thermosetting resin and has a columnar shape.
The above-mentioned mold has been heated, and thus, the tablet 228 housed in the pod 220 is gradually melted into a sealing resin in liquid form. Then, the sealing resin in liquid form pressurized by a plunger 222 is fed to the cavity 214 via the runner 218 and a gate 216, so that the semiconductor element 204 and the island 202 are sealed by the sealing resin. Also, along with injection of the sealing resin, air in the cavity 214 is released to the outside through an air vent 244.
The manufactured circuit device 200 is shown in FIG. 11B. The island 202, the semiconductor element 204, fine metal wires 206 and leads 210 are resin-sealed with a sealing resin 208. Also, the back surface of the island 202 is wholly covered with the sealing resin 208 in order to secure withstand voltage and moisture resistance.
With the sealing method described above, however, the bottom surface of the island 202 cannot be covered in some cases. Specifically, referring to FIG. 11B, the sealing resin 208 covering the bottom surface of the island 202 is desirably thin so that heat produced by the semiconductor element 204 can be well dissipated to the outside via the island 202 and the sealing resin 208. The heat dissipation performance of the whole device improves when the thickness of the sealing resin 208 covering the bottom surface of the island 202 is of the order of 0.5 mm or below, for example. Now, refer back to FIG. 11A. In order to achieve such a thickness, the gap between the bottom surface of the island 202 and an inner wall of the lower mold half 226 needs to be set narrow in the resin-sealing process, but there is a possibility that the sealing resin cannot be fully filled in the gap. If there is a region with no sealing resin filled in it, this region will be a void, causing a defect.
A sealing method for avoiding such a problem is described in Japanese Patent Application Publication No. 2010-86993. Referring to FIG. 3 and a part explaining it in Japanese Patent Application Publication No. 2010-86993, a resin sheet 52 disposed on the bottom surface of a circuit board 22 is melted to achieve thin resin sealing on the bottom surface of the circuit board 22.
Specifically, first, the resin sheet 52 formed by compressing a resin material is disposed on a lower mold half 44, and the circuit board 22 is placed on the top surface of the resin sheet 52. The resin sheet 52 is then heated and melted by the lower mold half 44, thereby thinly covering the bottom surface of the circuit board 22.
By covering the bottom surface of the circuit board 22 with the resin sheet 52 in this manner, the bottom surface of the circuit board 22 can be thinly resin-sealed without generating a void.
In the resin-sealing method described in Japanese Patent Application Publication No. 2010-86993, the resin sealing is performed using: the resin sheet 52 prepared underneath the bottom surface of the circuit board 22; and a molding resin injected into the cavity in which the circuit board 22 is disposed.
Here, even when the resin sheet and the molding resin are made of the same material, a gap may be formed in the interface of the two resins if they are thermally cured at different timings. This causes a problem of deteriorating the withstand voltage and moisture resistance at the interface.
Specifically, since the resin sheet is in surface contact with the mold, heat is easily transmitted from the mold to the resin sheet, and therefore the melting and thermal curing thereof occur in an early stage of the sealing. Then, by the time the molding resin in liquid form is injected into the cavity, the resin sheet has already been thermally cured and lost its flowability so that the resin materials of these may not be mixed together at the boundary therebetween. Such a situation may lead to the generation of a gap in the boundary between the molten resin sheet and the injected molding resin.
The present invention has been made in view of the problem described above, and an object thereof is to provide a method of manufacturing a circuit device in which a circuit element can be resin-sealed with sealing resins formed integrally with each other.